1. Field of the Invention
This invention relates to balun circuits for coupling between balanced and unbalanced lines or devices in an electronic system. More particularly, this invention relates to a miniaturized multi-layer balun circuit for use in mobile communication devices such as portable telephones and cordless telephones.
2. Description of Related Art
Typically, a balun is used to couple a two-line balanced circuit, such as a cellular telephone circuit, to a single-line (unbalanced) circuit, such as an antenna circuit. The following references provide background information relating to baluns and are incorporated by reference herein in their entireties:
[1] U.S. Pat. No. 4,994,755 to Titus et al., entitled "Active Balun," Feb. 19, 1991; PA1 [2] U.S. Pat. No. 5,039,891 to Wen et al., entitled "Planar Broadband FET Balun," Aug. 13, 1991; PA1 [3] U.S. Pat. No. 5,574,411 to Apel et al., entitled "Lumped Parameter Balun," Nov. 12, 1996; PA1 [4] S. A. Maas, "Microwave Mixers", Artech House, pp 244-255; PA1 [5] U.S. Pat. No. 5,455,545 to Garcia, entitled "Compact Low-loss Microwave Balun," Oct. 3),1995; PA1 [6] U.S. Pat. No. 4,725,792 to Lampe, Jr., entitled "Wideband Balun Realized By Equal-Power Divider and Short Circuit Stubs," Feb. 16, 1988; PA1 [7] U.S. Pat. No. 4,460,877 to Sterns, entitled "Broad-Band Printed-Circuit Balun Employing Coupled Strip All Pass Filter," Jul. 17, 1984; PA1 [8] U.S. Pat. No. 5,497,137 to Fujiki, entitled "Chip Type Transformer," Mar. 5, 1994; PA1 [9] U.S. Pat. No. 5,025,232 to Pavio, entitled "Monolithic Multilayer Planar Transmission Line," Jan. 18, 1991; PA1 [10] U.S. Pat. No. 4,847,626 to Kahler et al., entitled "Microstrip Balun-Antenna," Jul. 11, 1989; and PA1 [11] U.S. Pat. No. 4,755,775 to Marczewski et al., entitled "Microwave Balun for Mixers and Modulators," Jul. 5, 1988.
The term "balun" is a contraction of balanced to unbalanced. A balun is a RF balancing network or electric circuit for coupling an unbalanced line or device and a balanced line or device for the purpose of transforming from balanced to unbalanced or from unbalanced to balanced operation, with minimum transmission losses and high impedance transformation ratio. A balun is normally used between equipment and transmission lines or between transmission lines and antennas. A balun can be used with an unbalanced input and a balanced output or, in the reverse situation, a balanced source and an unbalanced load. Baluns can be used to interface an unbalanced input with a balanced transmission line by dividing the signal received at its unbalanced terminal equally to two balanced terminals and by providing the signal at one balanced terminal with a reference phase and the signal at the other balanced terminal with a phase equal to the reference phase plus or minus 180.degree.. Baluns can be used to interface a balanced or differential input from a balanced pair of two unbalanced transmission lines providing output signals which are 180.degree. out-of-phase (odd-mode excitation) and an unbalanced load driven by a single-ended input signal. The balun combines the signals of the balanced input and provides the combined signal at an another port.
A balanced line has two very closely spaced current paths (usually wires), each displaying an equal impedance with respect to ground. At all physical points along the line, the currents in the two paths are equal in magnitude and opposite in direction. Because the two paths are very closely spaced in relation to the wavelength of the signal they carry, their electromagnetic fields cancel each other everywhere in space except in the immediate vicinity of the line. The balanced structure is usually needed in devices such as balanced raixers, modulators, attenuators, switches and differential amplifiers, since balanced circuits can provide better circuit-to-circuit isolation, dynamic range, and noise and spurious signal cancellation. A balanced load is defied as a circuit whose behavior is unaffected by reversing the polarity of the power delivered thereto. A balanced load presents the same impedance with respect to ground, at both ends or terminals. A balanced load is required at the end of a balanced transmission line to ensure that the currents in the line will be equal and opposite.
Depending on the implementation, baluns can be divided into two groups: active and passive. Active baluns are described in references [1] and [2] and are constructed by using several transistors (so-called active devices). Although active baluns are very small, they are not generally preferred for the following reasons. First, due to the employment of active devices, noise will be introduced into the system. Also. active devices tend inherently to waste power; this makes them quite disadvantageous in radio telephone systems. Additionally, the low-cost fabrication of active baluns is limited to semiconductor manufacture. Conversely, passive baluns are quite popular. Passive baluns can be categorized into lumped-type baluns, coil-type baluns, and distributed-type baluns.
Lumped-element-type baluns are described in references [3] and [4]. Lumped-element baluns employ discrete components that are electrically connected, such as lumped element capacitors and lumped element inductors. Advantages of lumped-element-type baluns include small size and suitability for low frequency range usage. On the other hand, the performance of lumped-element-type baluns is not good in high frequency ranges (several Ghz), because the lumped elements are very lossy and difficult to control. Also, the operational bandwidth of lumped-element-type baluns is small (&lt;10%, typically).
Coil-type baluns (trifilar transformers) are very popular in applications in the UHF band or lower frequency range. Shortcomings of the trifilar transformer include unacceptable lossiness in the frequency range higher than the UHF band, and barriers to miniaturization beyond a certain size.
There are many kinds of distributed-type baluns. The first type is the 180.degree. hybrid device described in references [4] and [5]. They are constructed by several sections of quarter-wavelength transmission lines and a section of half-wavelength transmission line. The drawbacks of the 180.degree. hybrid device are size, difficulty in achieving a high impedance transformation ratio, and limitation to a balanced pair of unbalanced outputs. A second type is the combination of a power divider and a 180.degree. phase shifter as described in references [6] and [7]. Since the 180.degree. phase shift is achieved by a half-wavelength length difference, the size is still too large. The third type is the well-known Marchand-type balun as described in references [8]-[11]. This type of balun has very wide bandwidth (multi-octave). Further, both the phase balance and the amplitude balance are excellent. Moreover, it can be applied not only in a balanced port (load) but also in a balanced pair of unbalanced transmission lines.
A Marchand-type balun is illustrated in FIG. 1, and its equivalent circuit is shown in FIG. 2. In FIG. 1, balun 10 is constructed by a substrate 12 having formed on one surface a transmission line structure defined by a top conductive strip 14 and interlevel conductive strips 16 and 17, separated by an interlevel dielectric layer 13. A ground plane electrode 18 is formed on the opposing planar surface of dielectric substrate 12. Top conductor 14 includes a relatively narrow section 14-1 and a relatively wide section 14-2. Interlevel conductor 16 underlies the top section 14-1, and interlevel conductor 17 underlies the top section 14-2. Top conductor 14 is continuous in length, while interlevel conductors 16 and 17 are separated by a central balance point gap G that centers on the transition between the 14-1 and 14-2 sections of top conductor 14. Interlevel conductors 16 and 17 are electrically isolated from one another and are connected through via-holes 15 to ground plane electrode 18. A load may be coupled across the balance point gap G via a pair of microstrip transmission line strips 16-1 and 17-1 extending from the balance point ends of respective interlevel conductor strips 16 and 17. Strips 16-1 and 17-1 thus constitute the balanced port BP. Input terminals may be connected across one of interlevel conductors 16 and 17 and the corresponding sections 14-1 and 14-2 of top conductor 14, to provide the unbalanced port UBP. The length of each of the interlevel conductor sections 16 and 17 is 1/4 .lambda.. The width of the top conductor section connected to the unbalanced port UBP controls the impedance transformation ratio. This section is equivalent to a 1/4 .lambda. impedance transformer. A drawback to this configuration is that the size is 1/2 .lambda.. In RF applications, this size is still too large. In reference [8], the size is reduced by a zigzag and spiral arrangement. Under such arrangement, the modified Marchand-type balun can be chip-sized. However, the discontinuities of the spiral and zigzag arrangement will introduce some losses.